Singers have to recognize both the rhythm and pitch of notes to be able to sing choral music. Often, problems arise in choral sight reading when singers have trouble with one or the other. Go through the piece just with the rhythm, using a common syllable like "do." Draw attention to rhythms that are different from similar sections of the work, particularly if the song is so common that you may be tempted to sing what is familiar rather than what is notated. Then go through the piece and identify any unusual or large intervals. Sing just those intervals one by one, then the intervals in the written rhythms, and then go back and put the rhythms and pitches together for the entire piece.
Singers in choirs often are surrounded by those who have similar voice parts. This results in difficulty hearing or listening to other parts, which can result in entire sections singing wrong notes or rhythms, falling behind or singing out of tune. Sing a part different from your own to remind yourself what you should hear in other sections. The easiest way to do this is just to have everyone sing one part at a time. You also can try assigning lines that are far apart, such as having soprano follow the bass line and vice versa. This is excellent practice in reading different clefs.
Usually, the soprano line carries the melody in choral music, but this is not always the case. Identify where the important lines are throughout the piece, and then sing the piece having only the people with those lines sing. This will let you understand when to bring out parts and how each part connects in the entire tapestry of the work. The advantage of this is that you will know the part to which you can listen if you get lost, because the melodies and countermelodies are easiest to find by ear.
No matter what exercise you use to practice your choral music, go through your music before you sing anything and mark any significant markings such as accidentals, tempo changes and modulations of key. When you do sing the music the first time, your markings will make it more likely that you will sing notes and rhythms correctly.
If you are having trouble singing expressively in sight reading, ignore all of the notes and rhythms of the piece and recite the text, exaggerating all of the emotional words (e.g., light, crushed, soared).